Influence of Calcination and Reduction Conditions of Ni-Al-LDH Catalysts for CO₂ Methanation

Abstract

CO₂ methanation offers a sustainable route to reduce greenhouse gas emissions by converting carbon dioxide into methane, a valuable renewable fuel. This exothermic reaction not only mitigates its environmental impact but also provides energy-efficient benefits, as the heat generated can be reused in industrial applications. In this study, CO₂ methanation was carried out in a continuous flow reactor with a CO₂:H₂ molar ratio of 1:4 and a gas hourly space velocity (GHSV) of 12,000 h⁻¹, using a Ni-Al-LDH catalyst with a molar ratio of 2.3. The research focused on how calcination and reduction conditions affect catalyst structure and activity. Characterization techniques such as BET, XRD, TPR, H₂-TPD, and CO₂-TPD revealed that these conditions significantly influence surface area, crystallinity, phase composition, and metal dispersion. A higher reduction temperature decreased the surface area and increased both the crystallite size and basicity. The findings highlight that thermal treatment play a crucial role in optimizing the catalytic properties of NiAl catalyst. The sample calcined at 600 °C showed greater activity at lower reaction temperatures, while the catalyst calcined at 400 °C performed better above 300 °C. Additionally, the evaluation of the effect of the reduction atmosphere during catalyst activation showed that H₂ is a more effective reducing gas at lower reaction temperatures, whereas biogas showed a better performance at higher temperatures. Importantly, XRD results showed the catalysts maintained their structural integrity post-reaction, with no significant carbon deposition in the H₂ atmosphere, confirming their potential for long-term application in CO₂ methanation.

1. Introduction

CO₂ methanation is a promising solution to address environmental, technological, and economic challenges related to greenhouse gas emissions and the energy transition. By converting CO₂ into methane, it enables carbon capture and reuse, mitigating climate change while producing a renewable energy source that can replace fossil fuels and balance the intermittency of solar and wind energy [1]. Methanation also supports power-to-gas technologies for large-scale energy storage, provides feedstock for the chemical industry, and improves the utilization of green hydrogen produced via water electrolysis [2,3,4]. Its energy efficiency, economic viability, and alignment with global decarbonization goals make it a key technology for the circular economy and sustainable development.

The CO₂ conversion reaction is a catalytic process that involves the conversion of CO₂ and hydrogen (H₂) into CH₄ and water (H₂O) over a suitable catalyst under low temperatures and pressures [5]. The chemical reaction can be represented as CO₂ + 4H₂ → CH₄ + 2H₂O and is known as the Sabatier reaction whose limitations are slow kinetics because the C=O bond energy is high (750 kJ/mol), andCO₂ dissociation is very difficult [5,6,7]. This exothermic reaction produces renewable methane, which can be integrated into the existing natural gas infrastructure or serve as a feedstock for various chemical processes. Furthermore, the generation of low amounts of by products and high conversion rates when using a catalyst, makes the methanation process a promising large-scale method [2,8].

The catalyst is a crucial component of methanation, which explains why numerous researchers have devoted their efforts to synthesizing and investigating various materials for the Sabatier reaction [9,10,11,12]. Noble metal catalysts consistently exhibit exceptional performance in CO₂ methanation [13]. Nevertheless, as economic considerations necessitate the exploration of cost-effective and readily available alternatives, Ni-based catalysts are employed due to their low cost and good performance [8,9,10,11]. The structure of Ni-based catalysts is a crucial factor influencing the performance of CO₂ methanation. Given the structure-sensitive nature of the reaction, Ni particle size significantly impacts catalyst effectiveness. However, nickel catalysts demonstrate lower activity at low temperatures relative to Rh and Ru-based catalysts. Additionally, the presence of CO as a byproduct can lead to the formation of nickel carbonyl species and carbon deposition on the catalyst surface, which can result in deactivation [9,14,15].

Various methodologies have been utilized in the development of nickel-based catalysts, with a focus on controlling basicity, appropriate nickel reducibility, and particle size, all of which are crucial for adsorption capacity and the CO₂ redox reaction [11,12]. The currently developed synthesis route involves the formation of layered double hydroxide (LDH), which contain mixtures of divalent/trivalent metals comprising polycations and feature a layered structure. LDHs have several interesting properties, such as their basic character, ion exchange capacity and memory effect by which their structure can be reconstructed via moisture after calcination [9,12,14].

In a previous work some LDH catalysts were prepared by our group via co-precipitation [9,16]. Martins et al. 2025 [9] show that modifying the amount of aluminum and nickel in the structure of LDHs can increase the surface area and dispersion of the catalysts, resulting in a greater conversion of CO₂ to methane. Ni-Al-LDH catalysts were evaluated in a broad Ni/Al range (from 1.51 to 3.06) for CO₂ methanation, where the Ni₇₀Al₃₀ catalyst showed the best performance and stability. According to Perez-Lopez (2006) [16], the reduction temperature significantly influences the activity and selectivity of the catalysts for the CO₂ reforming of methane. In contrast, the catalytic properties were practically independent of the calcination temperature, which primarily affects the surface area, which decreases as the calcination temperature increases.

Considering that calcination has a significant impact on the structure and composition of LDHs and greatly improves their adsorption performance, in this study, the effect of calcination temperature and reduction conditions (temperature and atmosphere) on the physicochemical and catalytic properties of the Ni₇₀Al₃₀ catalyst for CO₂ methanation was evaluated.

2. Results

2.1. Catalytic Characterization

The elemental analysis was performed as previously outlined [9], where the Ni/Al molar ratio obtained by ICP-OES was 2.32, very close to the nominal value (2.3), with a deviation of less than 1%. Furthermore, the weight % of the experimental results in XRF were 81.4 wt% and 18.6 wt% for Ni and Al, respectively, with deviation below 3%. These results revealed that the real values were very close to the nominal ones, demonstrating the reproducibility of the coprecipitation method.

Figure 1 presents the N₂ adsorption–desorption isotherms, which are used to evaluate the surface properties of the samples. According to the IUPAC classification [17], the isotherms for all samples calcined at different temperatures are categorized as Type IV, with an H3-type hysteresis loop. This formation is typical of hysteresis indicating capillary condensation, which is characteristic of mesoporous materials with pore sizes ranging between 2 and 50 nm.

The experimental data related to nitrogen adsorption and desorption, such as the BET surface area, the average pore size, and the average pore volume are presented in

Table 1. Physicochemical properties by N₂ physisorption of Ni-Al catalyst calcined at different temperatures, and average crystallite size.

Sample	SBET (m2 g−1)	Vpore (cm3 g−1)	Dpore (nm)	Crystallite Size (nm)
				Calcined *	Reduced **
NiAl 400 °C 8 h	308	0.38	19.14	3.0	4.4
NiAl 600 °C 6 h	278	0.36	4.79	3.7	6.7
NiAl 800 °C 2 h	234	0.34	6.04	4.4	7.2

* Determined for NiO from reflection at 43.2° (200). ** Determined for Ni⁰ from reflection at 44.5° (111).

. The BET analysis suggests that an increase in calcination temperature results in a decrease in the surface area. However, this change in temperature does not appear to affect the pore volume. This observation may be crucial to understand the optimization of catalysts for specific applications, in which the surface area is a determining factor in catalytic activity. Pinthong et al. (2019) [18] demonstrated in their studies with Mg-Al Layered Double Hydroxides that the BET surface area gradually decreased as the calcination temperature increased. The average pore size diameters of all samples ranged between 12 and 20 nm. Additionally, as the calcination temperature increased, the pore volumes decreased, which in turn resulted in an increase in the average pore size diameter.

The calcination of nickel hydrotalcites (Ni-LDHs) has a significant impact on their structural and catalytic properties. Calcination leads to the decomposition of the lamellar structure of hydrotalcite, resulting in the formation of mixed oxides of nickel and aluminum (Ni-Al-O) and this effect can be observed in the XRD results presented in Figure 2. The calcined samples present reflections at 2θ angles = 37.3°, 43.2°, and 62.8°, respectively, and can be identified as the (111), (200) and (220) crystal planes in the face-centered cubic crystalline structure of NiO [9]. However, Ni-Al-O spinels also exhibit a similar diffraction pattern to that of NiO, and the peak at 37.5° may be associated with the phases of Ni-Al-O spinels (NiAl₂O₄–normal and Ni₂AlO₄–inverse) [9,19]. There are no reflections that could be associated with other crystalline compounds such as simple metal hydroxides.

The crystallite sizes of the calcined materials were determined from the reflection of the (200) plane of NiO. The sample calcined at 800 °C exhibited the largest crystallite size (4.4 nm), while the sample calcined at 400 °C presented the smallest crystallite size (3.0 nm). Calcination can decrease the dispersion of nickel particles, leading to larger crystallite sizes [20,21]. Similarly, smaller crystallite sizes generally result in a larger surface area. This effect was confirmed by a BET analysis (Table 1), which showed that samples with larger crystallite sizes had smaller surface areas.

The three samples obtained by calcination at different temperatures and reduced at 600 °C presented the diffraction profile shown in Figure 2b. All samples presented a small peak at 36.6°, mainly in the sample calcined at a low temperature (400 °C), indicating that the mixed oxides were not fully reduced. The presence of metallic nickel was evident in all three samples after the reduction process, with peaks at angles of 44.4°, 51.5°, and 76.3°, corresponding to the (111), (200), and (220) crystal planes of Ni⁰, respectively. Clearly, the crystallinity of the samples increases with the calcination temperature. The increase in crystallite size after reduction can be explained by the exposure of the metallic area and grain growth. The formation of metallic nickel at lower temperatures, particularly in the sample calcined at 400 °C, can be attributed to the presence of weakly bound NiO species, which are more easily reducible.

The temperature-programmed reduction (TPR) method was employed to investigate the reducibility of the catalyst, which is strongly influenced by the interaction strength between the metal and the structure (Figure 3). The calcination temperature significantly affects the reduction behavior of catalysts [22,23]. The TPR profile for the sample calcined at 600 °C for 6 h exhibited the thinnest peak, indicating a more uniform distribution of particle sizes. In contrast, the samples calcined at 400 °C and 800 °C showed broader peaks, suggesting less uniformity and more dispersed particles.

The wide range of reduction temperatures indicates the presence of different nickel oxide species [9,23]. The profiles were refined through deconvolution, and the temperature ranges were used to determine the reduced species. Bulk NiO, being an easily reducible phase, is the first to undergo reduction at lower temperatures between 250 °C and 400 °C. Smaller particles with larger surface areas tend to reduce more rapidly due to the increased contact area with the reducing gas. This effect is evident in the sample calcined at 400 °C, which exhibited a smaller crystallite size and greater surface area, and this phase represents approximately 50% for this sample. The sample calcined at 600 °C did not show a reduction peak at temperatures below 500 °C, whereas the sample calcined at 800 °C contained approximately 20% of this phase.

The reduction peaks between 530 °C and 720 °C indicate a reduction in mixed Ni-Al oxides. These peaks are attributed to the reduction in the inverse spinel (Ni₂AlO₄) and normal spinel (NiAl₂O₄), corresponding to the third and four peaks, respectively. This result indicates that calcination at different temperatures significantly influenced the nickel species formed during the reduction, potentially leading to variations in catalytic performance.

The sample calcined at 600 °C exhibited higher reduction temperatures for all three reduction peaks, indicating more stable nickel species, compared to the samples calcined at 400 °C or 800 °C. Furthermore, this sample showed a larger metallic area, along with the largest dispersion.

Sikander et al. 2019 [24] evaluated the different reducible species in samples with varying nickel contents and found that NiO is reduced at temperatures below 500 °C, while NiO interacting with the support requires a higher temperature for a complete reduction. The reduction of the Ni²⁺ species depends on the demographic position of the Ni species, whether in a non-stoichiometric spinel or mixed-oxide matrix, which are reduced at lower temperatures compared to nickel in the spinel-phase (Ni,Mg)Al₂O₄.

The H₂-TPD profiles of the sample calcined at 400 °C for 8 h (Figure 4a) revealed three distinct regions of hydrogen desorption, indicating different metal-hydrogen interactions. Desorption at 191 °C is associated with hydrogen interacting with nickel reduced from NiO. Between 300 and 330 °C, hydrogen is adsorbed at active sites of NiAl spinels [25]. Above 600 °C, a broad peak indicates a strong hydrogen interaction with Ni-Al species. These observations are crucial for understanding catalytic behavior and optimizing catalyst performance in various reactions. The samples calcined at 600 °C and 800 °C did not exhibit a reduction peak at low temperatures in the TPR analysis, indicating there is no presence of bulk NiO in these materials. In the H₂ chemisorption analyses, these samples displayed only two desorption regions, which are characteristic of hydrogen’s interaction with nickel reduced from NiAl spinels. This is evidenced by the difference in the H₂ desorption profile compared to the sample calcined at 400 °C. Furthermore, the size of the crystallites can influence the interaction between the metal and the support. Larger crystallites may lead to less metal dispersion, affecting the strength of the hydrogen interaction with the metal and, consequently, the observed desorption temperature [25].

Based on the integration results of the obtained profile, the metallic surface area (S_Ni) and dispersion (γ_Ni) were estimated according to the previously described methodology [9,25], and presented in

Table 2. Total dispersion, metallic area, and H₂ consumption for reduced catalysts.

Catalyst	Relative Area (%) *				Total H2 Consumption (µmol/gcat) *	Metallic Area (m2/g) **	Total Dispersion (%) **
	Peak 1	Peak 2	Peak 3	Peak 4
NiAl 400 °C 8 h	4.9	48.6	12.5	34.0	178.6	22.61	4.06
NiAl 600 °C 6 h	0.0	5.2	74.1	20.7	96.5	23.41	4.21
NiAl 800 °C 2 h	10.8	10.4	31.0	47.8	177.8	12.51	2.25

* Obtained from H₂-TPR. ** Obtained from H₂-TPD.

. As observed, the total dispersion of metallic Ni decreases in the sample calcined at 800 °C, along with the metallic area, as a result of the sintering of dispersed nickel. These results show that the sample calcined at 800 °C has the lowest metallic area and least total dispersion, likely due to the sintering effect at higher temperatures. The sample calcined at 600 °C showed the highest metallic area and greatest dispersion, indicating a better distribution of nickel species.

As has already been clearly demonstrated in this work, the calcination process had a great influence on the Ni phases of the catalyst, presenting different reduction properties. The CO₂-TPD profiles of the samples after the reduction in a 10% H₂/N₂ atmosphere at 600 °C are shown in Figure 5. The results for the three samples showed four desorption peaks at similar temperatures, the deconvolution of these profiles were compiled in

Table 3. Total basicity and deconvolution of CO₂-TPD profiles for reduced catalysts.

Samples	Total Basicity [μmol/g]	Weak [μmol/g] *	Medium [μmol/g] *	Strong [μmol/g] *	Very Strong [μmol/g] *
NiAl 400 °C 8 h	53.91	11.88 (22)	10.93 (20)	16.02 (30)	15.08 (28)
NiAl 600 °C 6 h	44.41	11.62 (26)	10.31 (23)	18.74 (42)	3.75 (8)
NiAl 800 °C 2 h	55.84	11.24 (20)	14.05 (25)	19.30 (35)	11.24 (20)

* The relative fraction of the peak in the deconvolution is shown in parentheses.

. As reported in the literature, the basic sites on the catalyst surface can be classified into three types: weak basic sites located at about 160 °C, medium basic sites from 200 °C to 400 °C, and strong basic sites above 400 °C [26,27]. The basicity of mixed oxides derived from hydrotalcite is strongly dependent on the layer compositions, the presence of promoters, and the type of anions present between the interlayer spaces [28,29].

The presence of basic sites favors the adsorption of CO₂, facilitating its activation and subsequent surface reactions. Furthermore, control of basic sites is essential to favor the oxidation of carbon deposits that may form. In our previous study, we assessed that the molar ratio of nickel significantly impacted the basicity of the catalyst [9]. In the current work, all samples contained an identical amount of nickel, resulting in a similar basicity across the three samples. The sample calcined at 600 °C exhibited lower basicity, likely due to a more uniform and homogeneous arrangement of nickel oxides, as suggested by the TPR analysis. The first peak at 185–190 °C, observed in all three samples, corresponds to the desorption of CO₂ from weak Brønsted groups (⁻OH) [29]. The intermediate peak at 262–277 °C is due to bidentate carbonates formed in metal–oxygen pairs [26]. Desorption peaks above 430 °C are linked to CO₂ bound to low-coordination oxygen anions, indicating strong basic sites [7,26]. An additional peak above 600 °C is attributed to the completion of the dehydroxylation process and the decomposition of carbonate anions formed during CO₂ adsorption [30].

The distribution of basic sites plays a fundamental role in the adsorption and activation of CO₂ during methanation. In this study, CO₂-TPD analysis revealed the presence of weak, medium, strong, and very strong basic sites, each contributing differently to the catalytic process [7]. Weak basic sites, typically associated with surface hydroxyl groups, are responsible for reversible CO₂ adsorption and are essential for initiating the reaction pathway. Medium-strength sites, often related to bidentate carbonate formation on metal–oxygen pairs, are believed to stabilize key intermediates and enhance the reaction kinetics [31]. Strong and very strong basic sites, attributed to low-coordination oxygen anions, can promote deeper CO₂ activation, but may also lead to overly strong adsorption, potentially hindering desorption and reducing turnover frequency [25].

In addition, some authors demonstrated that the weak and medium basic sites are the key to enhancing the methanation activity catalysts [9,19,28,29,30,31,32]. Ren et al. (2024) [29] synthesized a series of Mg-Al hydrotalcites, which were calcined at various temperatures to prepare supports for Ni-based catalysts. The calcination temperature affected the distribution of basic sites, with higher amounts of weak and medium basic sites increasing CO₂ adsorption. This improved the catalyst activity and stability in CO₂ methanation.

2.2. Catalytic Tests

The CO₂ hydrogenation activity of NiAl catalysts calcined at different temperatures was evaluated at reaction temperatures between 200 and 400 °C, as shown in Figure 6a. The influence of the calcination temperature on the catalytic performance is evident. The activity of the NiAl-600 catalyst is higher than that of NiAl-400 and NiAl-800 at lower reaction temperatures. However, above 300 °C, the CO₂ conversion of NiAl-600 was similar to NiAl-800, whereas NiAl-400 showed greater conversion, indicating that NiAl-400 requires higher temperatures to be fully activated. Despite having the same Ni/Al molar ratio, the catalytic activity profiles agreed with the characterization results, which showed different surface areas, crystallite sizes, and reduction temperatures. The TPD-H₂ and TPR results, along with the crystallite size measurements, reveal significant insights into the behavior of NiAl catalysts calcined at different temperatures. The sample calcined at 400 °C for 8 h exhibits the highest total H₂ consumption and metallic area, indicating a well-dispersed nickel phase with smaller crystallites, which demonstrates that the greatest CO₂ conversions were achieved at reaction temperatures above 300 °C. As the calcination temperature increases to 600 °C and 800 °C, the crystallite size grows, leading to a decrease in metallic area and total dispersion. This sintering effect at higher temperatures results in fewer active sites for hydrogen adsorption, as evidenced by the lower total H₂ consumption in the sample calcined at 600 °C. The sample calcined at 800 °C shows a significant reduction in dispersion and metallic area, correlating with the larger crystallite size and the reduced interaction strength between the hydrogen and nickel. These findings underscore the importance of optimizing calcination conditions to maintain high levels of dispersion and catalytic activity.

There is no linear correlation between the activity (CO₂ conversion) and the specific surface area (S_BET), especially for reaction temperatures below 300 °C, as previously reported [16]. Above this temperature, the sample calcined at 400 °C showed greater activity, which may be related to its high S_BET. However, the catalyst activity must be related to the crystallite size, and in this case, the catalyst calcined at 400 °C presented the smallest crystallite size, which would explain the greater activity of this catalyst for reaction temperatures above 300 °C.

The NiAl-600 catalyst, which exhibited a balanced distribution of weak and medium basic sites, showed superior activity at lower temperatures, suggesting that these sites are more effective in promoting CO₂ conversion under mild conditions. In contrast, the NiAl-800 catalyst, with a higher proportion of strong basic sites, demonstrated lower activity, likely due to stronger CO₂ binding and the limited surface mobility of intermediates. These findings reinforce the importance of tailoring the basic site distribution to optimize the catalytic performance of LDH-derived materials.

Furthermore, a CH₄ selectivity of 100% was achieved with all catalysts over the entire temperature range.

After the CO₂ methanation reaction, the catalysts were characterized by XRD (Figure 6b) and the results indicate that there is no evidence of carbon deposition, whereas the crystalline structure was preserved and no new phase was detected in the NiAl-400, NiAl-600 and NiAl-800. The Ni reflections indicate that there was no sintering according to the crystallite sizes calculated by the Scherrer equation, determined from reflection of Ni⁰ at 44.5°. The crystallite size of the spent samples did not show any significant difference in relation to the samples reduced before the reaction (fresh samples). This observation indicates the absence of sintering, thereby ensuring the stability of the catalyst structure.

Temperature-programmed oxidation (TPO) was performed on the spent catalysts, as shown in Figure 7. This analysis aimed to identify and quantify carbonaceous species deposited on the catalyst surface, providing valuable insights into their stability and efficiency. The initial weight loss observed below 160 °C was attributed to the thermal desorption of water and CO₂ adsorbed during the reaction, with water being a primary product of the methanation process. Between 160 °C and 400 °C, a weight gain was recorded for all samples, corresponding to the reoxidation of metallic Ni⁰ crystallites that remained after the catalytic tests [16]. Differential thermal analysis (DTA) revealed slight variations in the peak reoxidation temperature, with the NiAl-800 catalyst showing the lowest degree of reoxidation. Above 400 °C, weight loss was associated with the oxidation of carbonaceous deposits.

The NiAl-400 catalyst exhibited the highest carbon deposition (2.31%), which correlates with its higher CO₂ conversion at elevated temperatures. In comparison, NiAl-600 and NiAl-800 showed lower mass losses of 1.58% and 1.76%, respectively. All values remained below 3%, in agreement with previous findings, confirming the favorable basic properties of the catalyst [9].

2.3. Effect of Reduction Conditions on Catalysts

The reduction step of the catalyst plays a crucial role in the catalytic activity for CO₂ methanation. This process helps to better dispersion of nickel particles, thereby increasing the active surface area available for the reaction [33]. Furthermore, as reported by various studies, the catalyst reduction can enhance resistance to coke formation [9,18,33,34]. However, it is important to note that reduction/activation at inappropriate temperatures may lead to the sintering of nickel particles, which in turn decreases the active surface area and, consequently, the efficiency of the catalyst [35].

In this research, the influence of the reduction temperature on the NiAl catalyst was evaluated by reducing at 400, 500, and 600 °C under hydrogen flow. The catalytic activity results indicate that the H₂ flow was more effective in the reduction process at low reaction temperatures compared to other gas mixtures (containing CH₄ and/or CO₂). Furthermore, the catalysts reduced at 500 and 600 °C showed equivalent results, while the catalyst reduced with biogas at 600 °C exhibited a better performance at higher reaction temperatures. The samples reduced with CH₄ and biogas showed a significant amount of coke on the catalyst surface (Figure 8b). Apparently, the carbon deposited on the catalyst surface during the reduction with CO₂ and CH₄ led to the catalyst’s deactivation. However, when reduced with biogas, a carbon degassing process occurs at high temperatures, enhancing its catalytic activity.

After the CO₂ methanation reaction, the catalysts were characterized by XRD (Figure 8c). The results indicate that their reduced crystal structure was maintained, with peaks identified with the symbol * corresponding to NiO. Additionally, the signal at 25.9 °C evidences carbon deposition in the sample reduced with CH₄. This reduction step resulted in a significant increase in crystallite size, as shown in

Table 4. Average crystallite size of spent catalysts under different activation conditions.

Temperature (°C)	Time (h)	Atmosphere Reduction	Crystallite Size (nm)
400	4	90 N2 + 10 H2	3.1
500	2	90 N2 + 10 H2	3.9
600	1	90 N2 + 10 H2	3.4
600	1	90 N2 + 10 CH4	9.8
600	1	90 N2 + 10 CO2	3.2
600	1	75 N2 + 15 CO2 + 10 CH4	13.1

, indicating sintering and coke formation on the catalyst surface. For the other reducing gases, the diffraction reflections for Ni⁰ (111) showed no sintering, as determined by the crystallite sizes calculated using the Scherrer equation, thereby ensuring the stability of the catalyst structure.

3. Materials and Methods

3.1. Catalyst Preparation

Ni₇₀Al₃₀-LDH catalyst was synthesized according to previously reported procedures [9,16,19]. In summary, co-precipitation was performed at 50 °C and pH 8.0 ± 0.1 continuously in a jacketed reactor using a nitrate solution (1 M) of Ni and Al, along with an alkaline solution (2 M) of Na₂CO₃ and NaOH (50/50 wt%) from Synth, Brazil both added dropwise. Nickel nitrate (Ni(NO₃)₂·6H₂O, 97% P.A.) and aluminum nitrate (Al(NO₃)₃·9H₂O, 98% P.A.) from Vetec, Brazil were used. After precipitation, the resultant solution was aged for 1 h at 60 °C under agitation, then washed with deionized water and filtered until the conductivity was below 50 µS. The material was dried overnight at 80 °C and sieved (32–42 mesh), yielding the as-prepared LDH. Calcination in synthetic air was carried out at 400 °C, 600 °C and 800 °C for 8 h, 6 h and 2 h, respectively, to form mixed oxides. For evaluation of reduction temperature, sample calcined at 600 °C was submitted at reduction temperatures of 400, 500 and 600 °C for 4 h, 2 h and 1h, respectively. In addition, different gas mixtures were used during the activation/reduction: H₂, CH₄, CO₂ and CH₄/CO₂.

3.2. Catalyst Characterization

The physicochemical properties of the calcined samples were determined using nitrogen adsorption–desorption isotherms, and the tests were performed with a pore and surface analyzer (Quantachrome 4200e). Prior to analysis, the samples were degassed under vacuum for 3 h at 300 °C, and the measurements were carried out using liquid nitrogen at −196 °C. The specific surface area was obtained via the BET method, while the micro- and meso-pore volumes, as well as the pore diameter, were determined using the t-plot and Barrett–Joyner–Halenda (BJH) models [9,19,32].

The crystalline phases were characterized through X-ray diffractometry (XRD) using a Mini Flex diffractometer (Rigaku, 30 kV, 10 mA) equipped with a Cu-Kα radiation source (λ = 0.154 nm). This comprehensive analysis included as-synthesized materials (LDH), fresh samples (calcined and reduced), and spent samples (post-catalytic tests). The average crystallite size was estimated from the XRD patterns using the Scherrer equation (Equation (1)) [9,19,36].

$$d={\displaystyle \frac{k\lambda }{\beta cos\left(\theta \right)}}$$

(1)

Where k = 0.9, λ is the Cu-Kα radiation wavelength (0.154 nm), β is the line broadening at half width of NiO (2 0 0) or Ni⁰ (1 1 1) facets, and θ is the corresponding angle.

Thermal analyses, including H₂ temperature-programmed reduction (H₂-TPR), CO₂ temperature-programmed desorption (CO₂-TPD), and H₂ temperature-programmed desorption (H₂-TPD) measurements, were conducted using a multipurpose instrument (SAMP3) equipped with a thermal conductivity detector (TCD). For H₂-TPR measurements, approximately 100 mg of the samples were loaded into a U-tube quartz reactor and pretreated via N₂ flow at 100 °C. The analyses were carried out from 100 to 850 °C (with a ramp rate of 10 °C/min) in a flow of 5% H₂/N₂ (30 mL/min) [9,19,32].

Prior to programmed temperature desorption characterization, the samples underwent reduction with 10% H₂/N₂ (100 mL/min) at 600 °C for 1 h. The basic properties of the samples were evaluated via CO₂-TPD analyses. Reduced samples (100 mg) were heated to 100 °C, purged with He flow (30 min), then switched to CO₂ (30 min, 30 mL/min) for the adsorption step, and purged with He (30 min). Finally, CO₂ desorption was carried out from 100 to 800 °C (10 °C/min), with He as the carrier gas (30 mL/min) [9,19,37].

H₂ chemisorption properties were evaluated via H₂-TPD analyses. Around 100 mg calcined samples were reduced at 600 °C for 1 h with 10% H₂/N₂ (100 mL/min) flow, purged in N₂ flow (30 min) at room temperature, switched to H₂ flow (1 h, 20 mL/min) for the adsorption step, and purged with N₂ (30 min, 30 mL/min). Lastly, H₂ desorption was carried out from 50 °C to 800 °C (10 °C/min), with N₂ as the carrier gas (30 mL/min). Surface metallic area and metal dispersion were calculated using Equations (2) and (3) [19,38]:

$$S_{{Ni}^0}\left(m^2{g}^{-1}\right)={\displaystyle \frac{\mathrm{Y}\times N_A\times F_S}{\mathrm{A}}}$$

(2)

$${\gamma }_{{Ni}^0}(\%)={\displaystyle \frac{Y\times F_S}{\frac{W_m}{M_m}}}\times 10$$

(3)

with Y representing the H₂ chemisorbed amount (mol/g_cat), N_A is the Avogadro number (6.023 × 10²³ atoms/mol), A is the number of surface Ni atoms located at a unit area (1.54 × 10¹⁹ atoms/m²), F_S is the stoichiometric factor (H₂/Ni = 2), W_m is the Ni metal loading (g_Ni/g_cat), and M_m is the Ni molar mass (58.69 g_Ni/mol) [9,19].

3.3. Catalyst Tests

Catalytic tests were conducted, as comprehensively detailed in previous studies [9,19,32]. Quartz wool was utilized to support the catalytic bed within a fixed-bed tubular quartz reactor, which was heated by an electric resistive furnace. Digital mass controllers (Sierra Instruments, Monterey, CA, USA) were employed to regulate gas flows. The outlet gases were analyzed online using a gas chromatograph (Varian Co., Model: Star 3600cx, Lexington, MA, USA) equipped with a thermal conductivity detector, a Porapak-T column, and nitrogen as the carrier gas.

The samples, typically 100 mg, were initially reduced in situ at 400, 500 and 600 °C for 4, 2 and 1 h, respectively, using a flow of 100 mL/min of reducing gas. During the potential reduction assessment stage, the gases used were hydrogen (10% in N₂), CH₄ (10% in N₂), CO₂ (10% in N₂), and biogas (comprising 15% CH₄ and 10% CO₂ in N₂).

All tests were conducted at atmospheric pressure with a total gas hourly space velocity (GHSV) of 60,000 mL/(gcat·h). Activity tests were carried out in stepwise mode with a feed composition of H₂:CO₂:N₂ = 4:1:15 (v/v, 100 mL/min), varying the temperature from 200 to 400 °C in 50 °C increments, and with five gas chromatography (GC) analyses per temperature.

$$X_{{CO}_2}\left(\mathrm{\%}\right)={\displaystyle \frac{{F_{{\mathrm{C}\mathrm{O}}_2}}_{\mathrm{i}\mathrm{n}}-{F_{{\mathrm{C}\mathrm{O}}_2}}_{\mathrm{o}\mathrm{u}\mathrm{t}}}{{F_{{\mathrm{C}\mathrm{O}}_2}}_{\mathrm{i}\mathrm{n}}}}\times 100$$

(4)

$$S_{{CH}_4}(\%)={\displaystyle \frac{{F_{{CH}_4}}_{out}}{{F_{{CH}_4}}_{out}+{F_{CO}}_{out}}}\times 100$$

(5)

$$Y_{{CH}_4}\left(\%\right)=X_{{CO}_2}.S_{{CH}_4}\times 100$$

(6)

To investigate the presence of carbon deposits in spent samples, temperature-programmed oxidation (TPO) was conducted using a thermobalance (SDT Q600, TA Instruments, Newcastle, DE, USA). The samples (10 mg) were heated from ambient temperature to 800 °C at a rate of 10 °C/min in a synthetic air flow (100 mL/min) [9,16,19].

4. Conclusions

The N₂ physisorption analysis revealed that increasing the calcination temperature decreases the surface area. The XRD analysis indicated that calcination leads to the formation of mixed nickel and aluminum oxides, with higher temperatures resulting in larger crystallite sizes and smaller surface areas.

The TPR profiles indicated that the calcination temperature significantly affects the reduction behavior of the catalysts, with the catalyst calcined at 600 °C showing the most uniform particle size distribution. CO₂ desorption (CO₂-TPD) highlighted the importance of basic sites in CO₂ adsorption, facilitating its activation.

Calcination and reduction of Ni-Al-LDH catalysts has been shown to significantly impact the structural and catalytic properties in CO₂ methanation.

The NiAl-600 catalyst showed higher activity at lower reaction temperatures, while the NiAl-400 catalyst performed better above 300 °C. All catalysts achieved 100% selectivity for CH₄ across the entire temperature range. Thermogravimetry revealed that the NiAl-400 catalyst had the highest carbon deposition related to the highest CO₂ conversion at high reaction temperatures.

Hydrogen proved to be a more effective reducing gas at lower reaction temperatures, while biogas showed a better performance at higher temperatures, indicating that the choice of reduction atmosphere can be tailored to the intended operating conditions.

These results highlight the importance of choosing the appropriate calcination temperature and reduction conditions to optimize the performance of Ni-Al-LDH catalysts in the methanation of CO₂. A detailed understanding of the structural and catalytic properties of these materials is essential for the development of more efficient and sustainable processes for the conversion of CO₂ to methane, contributing to climate change mitigation.